Stereoselectivity of the Radical Reductive Alkylation of Enamines: Importance of the Allylic 1,3-Strain Model

Radical addition to enamines using Bu₃SnH as reducing agent are reported (Schemes 2 and 4). The diastereoselectivity of these reactions was examined in different systems (Tables 1 and 2). Enamines derived from cyclic ketones such as cyclohexanone were alkylated with high diastereoselectivity with preferential formation of the cis-disubstituted cycloalkanes. In acyclic systems such as enamines derived from propiophenone and diethyl ketone, moderate to high stereoselectivities were observed in the H-abstraction step. A model based principally on minimization of allylic 1,3-strain (A^1,3 strain) was deduced from the experimental results and semi-empirical (AM1) calculations.